1. Technical Field
The invention relates to an integrated read-only memory, a method for operating said read-only memory and a method for producing an integrated read-only memory.
2. Related Art
As the integration density in microelectronics increases, the demand for large-scale integrated read-only memories is also increasing. These memories are used for example for on-chip storage of audio, graphics or video data.
Read-only memories are distinguished by the fact that the memory content is preserved even when the operating voltage is switched off. Such read-only memories are, in particular, also of programmable design (PROM). Programmable components therefor are for instance fuses, diodes or, alternatively, special MOSFETs having an additional so-called floating gate. The latter is charged during programming and thereby shifts the threshold voltage of the MOSFET. Since the floating gate is insulated all around with SiO2, the charge retention can be guaranteed for approximately ten years.
Over and above the programming function, there are read-only memory variants which are of erasable design (EPROM, EEPROM). The memory content can be erased by means of ultraviolet light in the case of EPROMs; the erase function is effected electrically in the case of EEPROMs.
Flash memories constitute a particular embodiment of erasable read-only memories. They are electrically erasable, in which case, rather than being able to erase the individual memory cells separately, it is only possible to erase a whole block on the chip at once. In this case, the erasure is effected by means of a single erase pulse that lasts a few seconds. The advantage in this case is that the memory chip, for erasure, does not have to be demounted and placed into an erase device.
Integrated memories are usually constructed in the form of arrays. So-called selection transistors are used to select individual memory elements, so that their content can be read out. Individual selection transistors are selected via word lines. In this case, the word lines are connected to the control electrodes of selection transistors. The memory content is read out via bit lines. Writing to or configuring memory cells usually requires additional lines for accessing the memory element. This enlarges the construction of integrated read-only memories and makes them more complicated to handle.
An article by C. P. Collier et al., Electronically Configurable Molecular-Based Logic Gates, Science, Volume 285, p. 391, 1999 discloses an electronically configurable connection having a molecular monolayer between two contacts produced lithographically. In this case, the contacts are formed as Al—Ti electrodes. Rotaxane molecules are used as the molecular layer.
The electrical behavior of this connection can be described as follows: if a layer is negatively polarized, then the current at the connection rises with increasing negative polarity. Such a treatment of the electrical connection changes the switching behavior to the effect that now only a current that is lower by a factor of 60 to 80 is measurable in the case of a negatively polarized layer than without the prior treatment of the connection with a positively polarized layer.
The connection may thus be understood as a switch which may have an open state (poorer conductivity) and also a closed state (better conductivity). The open state permits a current flow at negative voltage on account of a resonance tunnel effect in the rotaxane-electrode junction. The switch's transition from the open state to the closed state through application of a sufficient positive voltage is irreversible, with the result that a switch, once closed, can no longer assume an open state.
The connection is disclosed for use in logic circuits.
A further electronically configurable switch is disclosed in C. P. Collier et al., A [2]Catenane-Based Solid State Electronically Reconfigurable Switch, Science, Volume 289, p. 1172, 2000: the electrodes used are a polycrystalline silicon electrode, on the one hand, and a metal electrode, on the other hand. A molecular monolayer between the electrodes contains Catenane.
Operation of the switch exploits the effect that mechanically blocked, intermeshing molecular rings of the catenane are shifted relative to one another upon oxidation and subsequent reduction and the electrical properties of the switching connection are thereby changed. This voltage-controlled shift is reversible. The configuration is thus effected along a hysteresis loop. Depending on the previously applied configuration voltage, it is possible to observe a specific switching behavior upon application of a predetermined read voltage.
A further embodiment of a molecularly constructed switch is revealed in D. I. Gittins et al., A Nanometre-Scale Electronic Switch Consisting of a Metal Cluster and Redox-Addressable Groups, Nature, Volume 408, p. 67, 2000. Here, too, the electron transport is controlled by means of molecular paths. A bipyridinium compound is used as the molecular layer.
A metal-insulator-metal arrangement is proposed in A. Beck et al., Reproducible Switching Effect in Thin Oxide Films for Memory Applications, Applied Physics Letters, Volume 77, p. 139, 2000. An insulator oxide, for instance SrZrO3 or SrTiO3 or Ca2Nb2O7, is applied as an epitaxial or polycrystalline film onto an SrRuO3 film or a Pt film as electrode. The top electrode made of Au or Pt is applied onto the insulator via a Ti layer.
A read access to the switching arrangement is effected in a voltage range of −0.5 Volt to +0.5 Volt in the case of SrZrO3 as insulator doped with 0.2 Cr. The current/voltage relationship is approximately linear in this read voltage range. The current flow over this voltage range depends on the previous configuration of the insulator. The insulator is configured by application of voltages of +1 Volt or −1 Volt over a duration of 2 ms. Through application of the negative configuration voltage, the insulator flips into its low-impedance state and in this case has a resistance characteristic curve that differs significantly from the resistance characteristic curve after application of the positive configuration voltage. Through application of the positive configuration voltage, the insulator flips into its high-impedance state. The configuration is reversible.
The change in the resistance characteristic curves that is brought about by configuration voltage pulses is caused by a change between amorphous and crystalline insulator states.
G. Wicker et al., Nonvolatile, High Density, High Performance Phase Change Memory, www.Ovonyx.com reveals chalcogenide alloys that are configured by controlled heating and cooling. In this case, the application of a voltage pulse brings about a change between amorphous and crystalline states, and vice versa.
H. J. Gao et al., Reversible, Nanometer-Scale Conductance Transitions in an Organic . . . ”, Physical Review Letters, Volume 84, No. 8, p. 1780, 2000 uses a complex comprising 3-nitrobenzal malonitrile and 1,4-phenylenediamine as a layer whose conductivity can be changed on account of a change between crystalline and amorphous states.
U.S. Pat. No. 4,590,589 discloses an electrically programmable read-only memory having voltage-programmable structures produced in finished fashion, for the provision of predictable and selectable programming voltages.